Don Quixote Volume 2, Chapter 71
Sancho tells Don Quijote he is very disappointed that Altisidora didn't keep her promise to give him her six slips. He points out that even when a patient dies, a doctor still expects to get paid and Sancho suffers and sheds blood but gets nothing! From here on in, he is going to ask for money up front before he works a cure on anyone. Don Quijote agrees with him and says he will gladly pay him for the lashes for Dulcinea (though he hopes that doesn't interfere with the rules of the disenchantment). This thought cheers Sancho greatly and he asks Don Quijote how much he's willing to pay for each lash. Don Quijote, tells him to count what money he has (Sancho is the money holder) and figure out what Don Quijote can afford. Sancho decides twenty-five cents a lash to be fair; which will earn him eight hundred and twenty-five dollars. Don Quijote tells him if he does it soon -- he'll throw in another hundred dollars.
Sancho gets started that evening. Standing amid a grove of trees he actually gives himself eight lashes. Sancho calls out that he should have charged more and Don Quijote says he'll give him double. After that, Sancho switches to thrashing the trees and groaning miserably to aid in the deception. After a while, afraid that Sancho might kill himself before the goal is reached, he asks Sancho to stop since he has already done well over a thousand. Sancho says he'd like to get another thousand done tonight. Don Quijote comments on how Sancho is certainly in the mood tonight and allows him to go on till Sancho lets out such a yelp and hits the tree so loudly that Don Quijote comes running and says enough is enough -- for this evening. Sancho asks him to throw his cloak over his back and they call it a day.